Most of the long-range forecasts are predicting above-normal temperatures throughout the summer. Heat stress can have a negative impact on beef cattle reproductive performance in the summertime. Compared to feedlot cattle, cows on pasture are generally at a lower risk of dying from heat stress. However, poor reproductive performance is often a hidden outcome of heat stress in cow herds. Spring-calving cows are expected to get pregnant during some of the hottest times of the year. Heat stress can decrease the fertility of both the cow and the bull. Additionally, bulls can become less active breeding and cows may not have as strong of heats.
Cow fertility
During extreme heat, cows may not be able to maintain their core body temperature. Follicles in ovaries can be damaged due to excessive body temperatures. These damaged follicles will be subfertile when they are ovulated, sometimes even well into the fall. Even if cows were ovulating fertile follicles when the heat event occurred, they may not express enough estrus behavior because of the extreme temperatures. Cattle undergoing stress release cortisol, which will decrease estradiol, reducing heat expression in the cow.
If the cow did get bred and conceive, the embryo is at risk of embryonic death from heat stress due to decreased blood flow to the uterus as the cow tries to maintain body temperature and possibly experiences a reduction in progesterone production.
Bull fertility
Bulls can also have heat-related issues. If the bull cannot adequately control testicular temperatures, sperm production will be reduced and abnormal sperm will increase. Decreased motility of sperm cells is evident within two weeks of a heat event and may not return to normal for eight weeks. During high temperatures, bulls will seek shade and will be less active in seeking receptive females.
Prevention
Grazing cattle should have access to cool, clean drinking water and shade during the summer. Surface water can become stagnant and contaminated with feces if cows are allowed to stand in the water to cool off. Although cows can survive drinking this water and will not become dehydrated, they will not want to consume adequate amounts to deal with heat stress and maintain high reproductive performance.
Shade is critical to keep cows cool. Trees, shade structures or buildings can provide adequate shade during the heat of the day. Be careful of old buildings that do not have adequate ventilation. High temperatures can affect grazing behavior and rotational grazing systems may need to be adjusted.
It is also important to institute a good fly control program. Fly tags generally provide good control, but consider some supplemental oilers, dusters or spraying to keep cows from congregating from excessive fly pressure.
